The present invention relates to a hydraulic brake system with auxiliary energy assistance in general, and more particularly for use in automotive vehicles incorporating antiskid brake control devices.
There are already known hydraulic brake systems including a control piston and a valve device which are arranged between a master cylinder and several wheel cylinders, the control device supplying a pressure built up by pedal force in the master cylinder to the wheel brake operating cylinders in the event of failure of the auxiliary pressure.
One vehicular brake system of this type is known from German published patent application No. 22 02 998. Basically, this system comprises a master cylinder, a brake booster, and an actuating device. Inserted between a pressure source, the brake booster and the depressurized return reservoir are two valves which, on their part, correlate the pressure in the brake booster to the respective state of motion of the wheels. One of these valves which is designed as a multiple-way valve connects with an actuating cylinder. The pressure of the brake booster is supplied via this multiple-way valve to an annular chamber of the actuating cylinder and displaces an actuating piston in dependence upon the correlated pressure. The pressure built up in the actuating cylinder this way is fed to the wheel brakes. Another port of the actuating cylinder is in connection with the master cylinder.
When the auxiliary pressure supply is fully operative, the brake system operation is such that a braking pressure is built up in the actuating cylinder which is proportionate to the correlated pressure.
The master cylinder pressure is ineffective in this state of operation, since a corresponding valve in the actuating cylinder will be closed already after a slight movement of the actuating piston.
Upon failure of the auxiliary pressure, the actuating piston remains in its rest position in which this valve is open. The pressure built up in the master cylinder is supplied via the open valve to the wheel brakes.
It has to be regarded as a disadvantage of the device described that a separate actuating cylinder has to be provided for each wheel to be controlled and that there is a comparatively high consumption of auxiliary energy.
Furthermore, a brake system with an anti-skid control is known from German published patent application No. 24 43 545, wherein the wheel-cylinder pressure is built up via a sleeve seal of a master cylinder under failure-free operating conditions. This sleeve seal acts as a check valve inasmuch as its flow resistance is low, when the flow is directed from a hydraulic booster associated with the master cylinder to the wheel brake operating cylinders, and the sleeve seal accomplishes quick interruption of communication between the booster and the brake circuits connected to the master cylinder in the presence of danger of inversion of flow as a result of a failure of the auxiliary energy, for example. In the case of such failure, the sleeve seal travels across a compensating bore during the braking operation and thus renders development of pressure in the master cylinder and static actuation of the wheel brakes connected to the master cylinder possible.
In this device, it has to be regarded as a disadvantage that the sleeve seal is required to constantly travel across the compensating bore upon failure of the auxiliary pressure and is, therefore, subject to wear.
Besides, a brake system constructed in such a manner affords no possibility of detecting the functional reliability of the sleeve seal with the auxiliary energy supply intact. Only in the event of failure of the auxiliary energy is it possible to recognize a damage to the sleeve seal inasmuch as then no braking pressure can be produced by depression of the brake pedal. Obviously at this time it may be too late to do anything to deal with this problem.
In addition, it has to be considered to be a disadvantage of the described brake system that the edges of the sealing lips of the sleeve seal are drawn into the compensating bore under certain pressure conditions in the master cylinder and in the booster and the amount of wear is increased many times over.